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The Electrochemical Gradient Advanced
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
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Printed: December 21, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
www.ck12.org
C HAPTER
Chapter 1. The Electrochemical Gradient - Advanced
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The Electrochemical
Gradient - Advanced
• Describe the electrochemical gradient.
Do you really have electricity flowing through your body?
Yes you do. These electrical signals allow information to flow through the nervous system extremely rapidly. And it
all starts with the formation of an electrochemical gradient.
The Electrochemical Gradient
The active transport of ions across the cell membrane causes an electrical gradient to build up across this membrane.
The number of positively charged ions outside the cell is usually greater than the number of positively charged ions
in the cytosol. This results in a relatively negative charge on the inside of the membrane, and a positive charge on
the outside. This difference in charges causes a voltage to exist across the membrane. Voltage is electrical potential
energy that is caused by a separation of opposite charges, in this case across the membrane. The voltage across
a membrane is the membrane potential. Membrane potential is very important for the conduction of electrical
impulses along nerve cells. The membrane potential of a cell at rest is known as its resting potential, and is
discussed below. A non-excited nerve cell is an example of a cell at rest.
Because of the ion gradient, there are less positive ions inside the cell, the inside of the cell is negative compared
to outside the cell. This resulting membrane potential favors the movement of positively charged ions (cations) into
the cell, and the movement of negative ions (anions) out of the cell. So, there are two forces that drive the diffusion
of ions across the plasma membrane—a chemical force (the ions’ concentration gradient), and an electrical force
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(the effect of the membrane potential on the ions’ movement). These two forces working together are called an
electrochemical gradient.
The electrochemical gradient determines the direction an ion moves by diffusion or active transport across a membrane. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is
also known as a proton motive force, due to both the proton gradient and voltage gradient across the membrane.
This potential energy is used for the synthesis of ATP by oxidative phosphorylation.
The Resting Potential
In order to maintain the membrane potential, cells maintain a low concentration of sodium ions (Na+ ) and high
levels of potassium ions (K+ ) within the cell (intracellular). The sodium-potassium pump moves three Na+ ions out
of the cell and brings two K+ ions into the cell. This essentially removes one positive charge from the intracellular
space. The resulting membrane potential is known as the resting potential.
FIGURE 1.1
This diagram shows how ions maintain the membrane potential.
The
sodium-potassium pump is shown in the
membrane, transporting three Na+ ions
(green) out of the cell and bringing two K+
ions (blue) into the cell.
The Ion Gradient
The electrochemical potential across a membrane determines the tendency of an ion to cross the membrane. The
membrane may be that of a cell or organelle or other sub cellular compartment. The electrochemical potential arises
from three factors:
1. the difference in the concentration of the ions on either side of the membrane,
2. the charge of the ions (for example Na+ , Ca++ , Cl− ), and
3. the difference in voltage between the two sides of the membrane (the transmembrane potential).
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Chapter 1. The Electrochemical Gradient - Advanced
Cotransport of ions by symporters and antiporter carriers is commonly used to actively move ions across biological
membranes. Transmembrane ATPases are often involved in maintaining ion gradients. The Na+/K+ ATPase uses
ATP to build and maintain a sodium ion gradient and a potassium ion gradient.
Proton Gradients and ATP synthase
One particular ion gradient with biological significance is the proton (H+ ) gradient. This type of gradient is
established through active transport involving proton pumps. The proton gradient is used during photosynthesis
and cellular respiration to generate a chemiosmotic potential, or proton motive force. This potential energy is used
for the synthesis of ATP by oxidative phosphorylation. The proton gradient can also be used to store energy for heat
production and flagellar rotation.
The energy held within the proton gradient can be used to synthesize ATP. ATP synthase is a transmembrane enzyme
that provides energy for the cell to use by producing ATP. The protein has two distinct regions, F0 and F1 . The F0
domain is embedded within the membrane, while the F1 domain is above the membrane, inside the matrix of the
mitochondria, or the stroma of the chloroplast. The F0 region is the proton pore, allowing hydrogen ions to diffuse
across the membrane. The F1 region of the protein has ATP synthesis activity, catalyzing the formation of ATP from
ADP and inorganic phosphate. Hence, ATP synthase is both an ion channel protein and enzyme. The synthesis
reaction is driven by the proton flow, which forces the rotation of a part of the enzyme; the ATP synthase is a rotary
mechanical motor. Bacteria may also have a version of this enzyme, where it, of course, is embedded in the cell
membrane.
During electron transport within the mitochondria (during cellular respiration) or chloroplast (during photosynthesis)
(discussed in the Concept Metabolism (Advanced) concept), a proton gradient is formed when protons are pumped
across the membrane by active transport. These hydrogen ions flow back across the membrane by facilitated
diffusion through ATP synthase, releasing energy which is then used to convert ADP to ATP (by phosphorylation).
Chemiosmosis is the diffusion of protons across the biological membrane through ATP synthase, due to a proton
gradient that forms across the membrane during electron transport.
Vocabulary
• ATP synthase: Ion channel and enzyme complex; chemically bonds a phosphate group to ADP, producing
ATP as H+ ions flow through the ion channel.
• chemiosmosis: Process in cellular respiration or photosynthesis which produces ATP; uses the energy of
hydrogen ions diffusing through ATP synthase.
• chemiosmotic potential: A difference in concentration of hydrogen ions across a membrane within the
mitochondrion or chloroplast; established using energy from an electron transport chain; also known as a
chemiosmotic gradient.
• electrochemical gradient: Difference across a membrane due to both a chemical force and an electrical force;
drives the movement of ions across the membrane.
• membrane potential: The voltage difference across a membrane; the basis for the conduction of nerve
impulses along the cell membrane of neurons.
• oxidative phosphorylation: A metabolic process that uses energy released by the oxidation of nutrients to
produce adenosine triphosphate (ATP).
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• proton gradient: Gradient established from a higher concentration of protons on one side of a membrane
compared to the other side of the membrane.
• proton motive force: The storing of energy due to a combination of a proton gradient and a voltage gradient
across a membrane.
• resting potential: The membrane potential of a cell/neuron at rest; the membrane potential of an unstimulated
neuron.
• voltage: The difference in electrical potential energy of two points/areas; electrical potential energy that is
caused by a separation of opposite charges.
Summary
• The voltage across a membrane is the membrane potential and the membrane potential of a cell at rest is the
resting potential.
• The electrochemical gradient is composed of a chemical force (the ions’ concentration gradient) and an
electrical force (the effect of the membrane potential on the ions’ movement).
• Chemiosmosis is the diffusion of protons across the biological membrane through ATP synthase, due to a
proton gradient that forms across the membrane.
Explore More
Use this resource to answer the questions that follow. Gradients at http://www.youtube.com/watch?v=kQ_3mI0WY
i0
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/139344
1. Why does an electrochemical gradient form across a cell membrane?
2. Why are positive ions attracted to the inside of a cell?
3. How do ions flow in and out of a cell?
Review
1. Define the electrochemical gradient.
2. Describe the role of ATP synthase.
3. What is chemiosmosis?
References
1. Image copyright Alila Medical Media, 2014. Ionic basis of resting membrane potential . Used under license
from Shutterstock.com
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